Have you bult up an engine before?
If not, I would recommend at least Des Hamills book;-
http://www.amazon.co.uk/Power-Rover-Eng ... 1901295079 (note that this is an earlier edition, the later one has a different cover)
Although this is title as a go fast book, which is covered, it will also cover all the aspects that need to be checked and how to check them. It covers what the workshop manuals don't. I think it is a very good book. Workshop manuals are generally written for trained mechanics and they assume that you know all the 'basics' so they skip right over these. Vizards books are also good, but he will (for example) say 'make sure the piston to bore clearance is xyz' but does not show you how to check it. Des Hamils book does.
The other one to get is Vizards;-
http://www.bevenyoung.com.au/pr451.htm
One aspect to keep in mind with any books in respect of go fast stuff is that this is these guy's experiences, so keep an open mind and I would tend to look upon them (in particular Des Hamils book) as starting points with a fairly sound basis.
Note also that these books only cover the engine and one touches on clutches, but they don't cover gearboxes, final drive or anything else.
Craig Wattson sells both of these so why not support a fellow ausmini member?
Also look on line at sites like Mini Mania and Mini Spares.
Look at what is available (to make your mini go better), then have a realistic think about what you would like and what can realistically be achieved, and look at your budget. Make some allowances for stuff that you haven't thought of + a dyno session at the end as if the motor is non standard this will be the only way to set your carbuation and graph the distributor etc......
Anyhow, these are just my thoughts..........
<EDIT> Just another 'rememberance'. DV's book is good from the aspect of the info in it, but it is a difficult read as it really should have been better seperated in to more sub-sections (try getting through the chapters on cylinder heads!)